This invention relates to compositions of matter classified in the art of chemistry as salicylate esters, compositions containing them, their combination with polyisocyanates, and the use of said combinations as pot life extenders for hydroxy terminated polybutadiene based polyurethanes useful as binders in propellant grains for solid fuel rocket motors.
Polyurethane binders may be used to prepare solid propellant grains with superior physical properties. It is advantageous to use hydroxy terminated polybutadienes as prepolymers for these binder systems. These prepolymers consist of a backbone of repeating polybutadiene units having terminal hydroxyls and an average molecular weight of about 3000. Additional hydroxyl functions are randomly located along the chain in such fashion that the average hydroxyl functionality per average 3000 molecular weight is in the range of from 2.1 to 2.7. The additional hydroxyl groups provide cross-linking sites necessary for firm cures, as the typical common polyisocyanates employed in the curing reaction are difunctional.
Binders may also include, in addition to prepolymers and curing agent, a plasticizer. This will be included to enhance such desirable physical properties as strain capability, softness, flexibility and the like. Plasticizers, being normally of lower viscosity than the prepolymer may also be useful in improving processing characteristics of the propellant during mixing and casting. Usually such common plasticizers as alkyl esters, for example, dioctyl adipate, or dinonyl phthalate, are employed although dual purpose materials such as n-butyl ferrocene which is also a burn rate catalyst may be employed.
The solid ingredients of the propellant will normally be an oxidizer or oxidizers, for example, ammonium perchlorate, or ammonium nitrate, a metal fuel, for example, aluminum powder, and frequently a solid burn rate catalyst, for example, iron oxide. The blending, casting and curing of these propellant grains is, of course, a procedure quite familiar to those skilled in the art.
The time required for manufacture of a solid propellant grain is, of course, limited generally by factors common to all plastic resin processes, those of pot life and cure time. Pot life in this instance is practically defined as the time required for the mix to reach a viscosity in the range of about 40 Kp. After this viscosity limit is reached, defect free casting can no longer be assured. In the usual case useful pot life will be about 10 to 14 hours. The time for cure to a rubbery state which permits motor tooling removal, and finishing operation is, as would be expected, dependent upon specific formulations and cure conditions but is generally of the order of 7 to 14 days.
Pot life and cure time are dependent primarily on the speed of the reaction between the hydroxyl terminals of the prepolymer and the polyisocyanate. Excessive pot life and cure time may be shortened by the use of conventional urethane cure catalysts in the mix. Typical are such catalysts as tertiary amines, metal salts or complexes, and organo tin compounds, for example, dibutyl tin diacetate. The expedient of raising the temperature to speed cure, which might at first glance seem attractive, must be applied cautiously because of the highly energetic nature of the materials involved and the necessity of avoiding excessive strain in the cured grain caused by cool down to ambient temperature after processing. Normally temperatures in the region of 63.degree. C are employed for cure, and seldom, if ever, would temperatures above 76.degree. C be permissible.
It is more common to encounter situations where, in order to obtain a cure rate within the acceptable range, the pot life is excessively shortened. This condition may also be encountered where high solids loading produces an inherently high viscosity. Remedies for this condition are inherently more difficult to obtain. High solids loading for certain applications cannot be reduced because for high burn rates oxidizer quantity and particle size must remain invariant. Processing aids, which are generally surface active materials, may reduce viscosity slightly and lengthen pot life. An inherently fast cure reaction may be somewhat modified by additives but the solution very often is only to change to other less reactive polyisocyanates as cure reagents. These frequently are more expensive, or impart less desirable physical properties to the cured grain.
It has been known in other urethane processing areas that the reactivity of polyisocyanates may be modified by treating them with a "blocking agent" such as phenol. The reaction between the isocyanate and phenol leads to a urethane which is stable at lower temperatures but which dissociates in a reverse reaction at temperatures greater than 160.degree. C regenerating the isocyanate and the phenol.
Because of the temperature required for reversion it is obvious that phenol itself cannot be used as a blocking agent in the manufacture of propellant grains.
A number of modified phenols have been reported to give lower unblocking temperatures.
U.S. Pat. No. 3,317,463 teaches that the use of alkyl and aryl p-hydroxy benzoates as blocking agents will enable unblocking to occur at temperatures of from 60.degree. to 110.degree. C.
U.S. Pat. No. 3,798,090 teaches that nitrocellulose bound propellants are curable with isocyanates blocked with phenols substituted with "negative groups," that is, nitro, nitroso, cyano, bromo, chloro, iodo, chloromethyl, dichloromethyl, trichloromethyl, ester, keto and the like, which will unblock at appropriate cure temperatures (about 60.degree. C).
Marchenko et al in "Effect of the Structure of Urethanes on Their Dissociation Temperatures" in Soviet Urethane Technology Chapter 31, Technomic Publishing Co., Westport, Conn. (1973), discuss the relationship between structure and dissociation temperature of a series of monomeric urethanes of the general structure ##STR1## wherein R was hydrogen, ortho-, meta; or para-methyl, or para-methoxy, and R' was hydrogen, ortho-, meta-, or para-nitro, ortho-, or para-fluoro, ortho-, or para-bromo, para-chloro, or ortho-methoxy. Other compounds and parameters related to dissociation are also discussed.
There is no suggestion in any of these references that salicylate ester blocked polyurethane prepolymers will have the requisite stability within the restrictive temperature ranges suitable for polyurethane based propellant processing, while dissociating within the similarly restrictive range of temperatures suitable for their cure reaction.